Nitrogen retention of biochar derived from different feedstocks at variable pyrolysis temperatures

Pyrolysis temperature is known to affect the potential of biochar as a nutrient-holding soil amendment, but the mechanism is unclear. In this study, physiochemical properties of biochar derived from switchgrass, water oak and biosolid at different pyrolysis temperatures were characterized using multiple techniques. In addition, biochar sorption of nitrogen nutrients in forms of ammonium (NH4+), nitrite (NO2−) and nitrate (NO3−) was investigated. The results indicated that the profound effects of pyrolysis temperature on biochar physiochemical properties were feedstock-specific. Similarly, biochar derived from different feedstocks at different temperatures exhibited different strengths to retain NH4+, NO2− and NO3−. The maximum NH4+ retention capacities were found with the biochar of switchgrass produced at 800 °C (10.47 mg g−1), biochar of water oak at 400 °C (3.82 mg g−1), and raw biosolids (43.29 mg g−1). Meanwhile, biochar was found to have limited (switchgrass and water oak) and negative (biosolids) retention for NO2− and NO3−. The cation exchange capacity (CEC) and morphological characteristics (e.g., porosity) were found to be the predominant factors affecting biochar NH4+ sorption.